There is no currently no actual evidence that TikTok is a threat to national security, but consider this:
TikTok is owned by Chinese company
All large Chinese companies are required to have Party members in the board, making them defacto state-run
PR China is currently committing genocide, large-scale espionage, subversion of foreign countries and all sorts of fun stuff associated with authoritarian fascist dictatorships
Fun fact: The waiting time for organ transplants in China is about an order of magnitude shorter than in the rest of the world. This is purely because of China's large population and not because of any other reason at all.